TWI399295B - Polyethylene multi-layer microporous film and separate for battery using the same, and battery - Google Patents

Description

Polyethylene multilayer microporous membrane and battery separator and battery using same

The present invention relates to a polyethylene multilayer microporous membrane, and a battery separator and battery using the same, the polyethylene multilayer microporous membrane having excellent penetration, mechanical properties, dimensional stability, shutdown characteristics, melting characteristics And the balance between compression resistance and electrolyte absorption.

The polyolefin microporous membrane is widely used in battery separators for lithium batteries, separators for electrolytic capacitors, various films, moisture-permeable waterproofing materials, and various filtration membranes. When a polyolefin microporous membrane is used as a separator for a battery, its performance is closely related to the characteristics, productivity, and safety of the battery. In particular, in addition to excellent mechanical properties and penetrability, the separator for lithium batteries is also required to prevent battery heat, fire, cracking, etc. caused by short circuit or overcharge of external circuits. The performance (closed characteristic) of causing the battery to stop when the pores are clogged, or the shape can be maintained even at a high temperature, and the performance of the dangerous substance (size stability) can be prevented by directly reacting the positive electrode material with the negative electrode material.

Therefore, there is a proposal to disclose a separator for a battery (Japanese Patent Laid-Open No. 62-10857), which is composed of a first layer and a second layer, which is composed of (a) a polymer composition (for example, polyolefin + At least one microporous sheet having a thickness of less than 0.025 cm, which is composed of a filler such as a metal oxide, and which is substantially non-porous at a temperature of about 80 to 150 ° C; The two layers are composed of (b) a polymer composition, and at least one microporous sheet having a thickness of less than 0.025 cm and having at least 25% by volume of pores (average pore diameter: about 0.005 to about 5 μm). It is configured to maintain the fine pore structure and size at a normal temperature to a temperature higher than the non-porous temperature of the first layer by about 10 °C. The battery separator has excellent dimensional stability and shutdown characteristics.

Japanese Laid-Open Patent Publication No. Hei 11-329390 discloses a battery separator, which is excellent in shutdown characteristics and strength. The battery separator is composed of two layers of fine polyolefin porous layer and a sandwich between them. The filler is made of a polyethylene separator, and the filler-containing polyethylene separator is composed of a fine porous film made by a particle stretching method.

JP-A-2002-321323 proposes a polyolefin microporous film which is excellent in safety function and strength, and is useful as a polyolefin microporous film useful as a separator for batteries, and is microporous with polyethylene and polypropylene as essential components. The film A and the polyethylene microporous film B are laminated and integrated. A preferred laminated structure of the above microporous film is film A/film B/film A or film B/film A/film B, as disclosed in JP-A-2002-321323.

However, recently, the spacer material not only requires shutdown characteristics, mechanical strength, and dimensional stability, but also requires characteristics such as battery durability, such as characteristics of durability of the battery and electrolyte absorption, such as improvement in cycle characteristics. In particular, the electrode of the lithium ion battery expands due to the insertion of lithium during charging, and the lithium detaches and contracts during discharge. With the recent increase in the capacity of the battery, the expansion ratio during charging tends to increase. Since the separator is pressed when the electrode is expanded, it is required that the separator has a property of being less resistant to penetration due to compression (resistance to compression). When the compression resistance of the microporous film is poor, when used as a battery separator, there is a possibility that the capacity of the battery is insufficient (the cycle characteristics are deteriorated).

Therefore, the applicant of the present application (Japanese Laid-Open Patent Publication No. 2004-149637) discloses a microporous film comprising a polyolefin and a non-polyolefin thermoplastic resin (for example, a microporous film of polybutylene terephthalate). The fibrils formed by the fine crack-like voids which are dispersed in the polyolefin and have fine particles having a diameter of 1 to 10 μm (mainly composed of a non-polyolefin-based thermoplastic resin) and are cracked. The structure of the above-mentioned fine particles is maintained in the pores. The present inventors have also proposed a microporous film comprising (a) polyethylene, and (b) a non-polyolefin thermoplastic resin (for example, poly. Methylpentene-1), which has a melting point or glass transition point of 170 to 300 ° C, and is slightly dispersed in a manner that is completely undissolved when melted and kneaded with polyethylene and its solvent, at a pressure of 5 MPa, 90 ° C The air permeability after the heat compression for 5 minutes is increased to 500 seconds/100 cubic centimeters or less. However, the electrolyte absorption and compression resistance of the microporous membranes of JP-A-2004-149637 and JP-A-2004-161899 cannot be said to be sufficient. .

Accordingly, it is an object of the present invention to provide a polyethylene multilayer microporous film having excellent penetration, mechanical properties, dimensional stability, shutdown characteristics, melting characteristics, and balance between compression resistance and electrolyte absorption as a battery It is useful to use spacer materials.

In view of the above object, the inventors have found that a polyethylene multilayer microporous film can be obtained only by adding a heat resistant resin and a filler to an inner layer of a polyethylene multilayer microporous film composed of at least three layers. It is not only excellent in compression resistance, but also excellent in electrolyte absorption (absorption speed and absorption), and the present invention is considered.

That is, the polyethylene multilayer microporous film of the present invention is characterized in that the polyethylene multilayer microporous film composed of at least three layers has: (a) a first porous layer composed of a polyethylene resin. And forming a surface layer of at least two sides; and (b) the second porous layer comprising a polyethylene resin, a heat-resistant resin having a melting point or a glass transition temperature of 150 ° C or higher, and a filler, at least sandwiched between the two skin layers layer.

The polyethylene-based resin of the first and second porous layers is made of ultrahigh molecular weight polyethylene having a mass average molecular weight of 5 × 10 ⁵ or more, and Mw of 1 × 10 ⁴ or more to less than 5 × 10 ⁵ The composition of the density polyethylene is preferably composed. The heat resistant resin is preferably at least one selected from the group consisting of polyester, polymethylpentene, and polypropylene. The above polyester is preferably composed of polybutylene terephthalate. The foregoing filler is preferably an inorganic filler.

The separator for a battery of the present invention is formed using the above polyethylene multilayer microporous film.

The polyethylene multilayer microporous film of the present invention has excellent balance of penetrability, mechanical properties, dimensional stability, shutdown characteristics, melting characteristics, and compression resistance and electrolyte absorption (absorption speed and absorption amount). By using such a polyethylene multilayer microporous film as a separator, it is possible to obtain a battery which is excellent not only in capacity characteristics, cycle characteristics, discharge characteristics, but also in safety and productivity such as excellent heat resistance and compression resistance.

[1] Polyethylene multilayer microporous membrane

The polyethylene multilayer microporous membrane is a polyethylene multilayer microporous membrane composed of at least three layers, and has: (a) a first porous layer composed of a polyethylene resin to form a surface layer of at least two sides; b) The second porous layer contains a polyethylene resin, a heat-resistant resin having a melting point or a glass transition temperature (Tg) of 150 ° C or higher, and a filler, and at least one layer is interposed between the two surface layers.

(A) first porous layer (1) Polyethylene resin

The polyethylene-based resin forming the first porous layer is preferably a composition composed of ultrahigh molecular weight polyethylene and polyethylene other than the ultrahigh molecular weight polyethylene. The ultrahigh molecular weight polyethylene has a mass average molecular weight (Mw) of 5 × 10 ⁵ or more. The ultrahigh molecular weight polyethylene is not only a single polymer of ethylene, but also a small amount of an ethylene-α-olefin copolymer containing other α-olefins. The α-olefin other than ethylene is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate and styrene. . The Mw of the ultrahigh molecular weight polyethylene is preferably from 1 × 10 ⁶ to 15 × 10 ^{6 , more} preferably from 1 × 10 ⁶ to 5 × 10 ⁶ . The ultrahigh molecular weight polyethylene is not limited to a single substance, and may be a mixture of two or more types of ultrahigh molecular weight polyethylene. The mixture may, for example, be a mixture of two or more kinds of ultrahigh molecular weight polyethylenes having different Mw.

The polyethylene other than the ultrahigh molecular weight polyethylene is selected from the group consisting of high density polyethylene having a Mw of 1×10 ⁴ or more to less than 5×10 ⁵ , medium density polyethylene, branched low density polyethylene, and chain low density poly At least one of the groups consisting of ethylene is preferred, and high density polyethylene is more preferred. The polyethylene having a Mw of 1×10 ⁴ or more to less than 5×10 ⁵ is not only a single polymer of ethylene, but also a copolymer of a small amount of other α-olefins such as propylene, butene-1 and pentene-1. . Such copolymers are preferably made by a single-sided catalyst. The polyethylene other than the ultrahigh molecular weight polyethylene is not limited to a single substance, and may be a mixture of polyethylene other than two or more kinds of ultrahigh molecular weight polyethylene. The mixture may, for example, be a mixture of two or more kinds of high-density molecular weight polyethylenes having different Mw, a mixture of the same medium-density polyethylene, and a mixture of the same low-density polyethylene.

The content of the ultrahigh molecular weight polyethylene in the polyethylene composition is preferably 1% by mass or more, and more preferably 10 to 80% by mass, based on 100 parts by mass of the total amount of the polyethylene composition.

The polyethylene resin is not limited to the above polyethylene composition, and only ultrahigh molecular weight polyethylene or polyethylene other than ultrahigh molecular weight polyethylene may be used as necessary. In either case, the Mw of the polyethylene-based resin is not particularly limited, and is usually 1 × 10 ⁴ or more, preferably 5 × 10 ⁴ to 15 × 10 ^{6 , and more} preferably 5 × 10 ⁴ to 5 × 10 ^{6 .} good. When the Mw of the polyethylene resin is 15 × 10 ⁶ or less, melt extrusion is easy.

The polyethylene resin may also contain a polyolefin having a melting point of less than 150 ° C other than polyethylene as necessary. Polyolefins having a melting point of less than 150 ° C other than polyethylene may be selected from polybutene-1, polypentene-1, polyhexene-1, polyoctene having a Mw of 1×10 ⁴ to 4×10 ⁶ . At least one of the group consisting of -1 and an ethylene-α-olefin copolymer and a polyethylene wax having a Mw of from 1 × 10 ³ to 1 × 10 ⁴ . Polybutene 1, polypentene-1, polyhexene-1, polyoctene-1 are not only individual polymers, but also copolymers containing other α-olefins. The content of the polyolefin having a melting point of less than 150 ° C other than polyethylene is preferably 20% by mass or less, and more preferably 10% by mass or less, based on the entire polyethylene resin.

The Mw/Mn of the polyethylene-based resin is not limited, and when the polyethylene resin is composed of any of the above polyethylene composition, ultrahigh molecular weight polyethylene, or polyethylene other than ultrahigh molecular weight polyethylene, 5~ 300 is better, preferably 10~100.

When the Mw/Mn is less than 5, the high molecular weight component is too large, and it is difficult to melt and extrude. When the Mw/Mn is more than 300, the low molecular weight component is excessive, and the strength of the microporous membrane is lowered. The Mw/Mn is a measure of the molecular weight distribution, and the larger the value, the larger the width of the molecular weight distribution. The Mw/Mn of polyethylene (individual polymer and ethylene-α-olefin copolymer) can be appropriately adjusted by multistage polymerization. The multistage polymerization process preferably comprises a two-stage polymerization in which a high molecular weight polymer component is produced in the first stage and a low molecular weight component is produced in the second stage. In the case of the polyethylene composition, the larger the Mw/Mn, the larger the difference in Mw between the ultrahigh molecular weight polyethylene and the polyethylene other than the ultrahigh molecular weight polyethylene, and vice versa. The Mw/Mn of the polyethylene composition can be appropriately adjusted by the molecular weight and the mixing ratio of each component.

(2) Composition of the two skin layers

The composition of the first porous layer forming the two skin layers may be the same or different, and the same is preferred.

(3) Number of layers

The first porous layer is usually required to have a surface layer on both sides, or may have three or more layers as necessary. For example, between the two skin layers, a first porous layer different in composition from the two surface layers is provided together with the second porous layer.

(4) The role of the first porous layer

When the two surface layers of the polyethylene multilayer microporous film are formed using the above first porous layer, the mechanical properties, the penetrability, the dimensional stability, the shutdown property, and the melting property of the polyethylene multilayer microporous film become good.

(B) second porous layer (1) Polyethylene resin

The polyethylene resin of the second porous layer may be the same as described above. However, the composition of the polyethylene-based resin of the second porous layer may be the same as or different from the composition of the polyethylene-based resin forming the first porous layer of the two surface layers, and may be appropriately selected depending on the desired physical properties.

(2) Heat resistant resin

The melting point or glass transition temperature (Tg) of the heat resistant resin is 150 ° C or higher. The heat resistant resin is preferably a crystalline resin having a melting point of 150 ° C or higher (including a partially crystalline resin) and/or a noncrystalline resin having a Tg of 150 ° C or higher. Here, the melting point and the Tg system can be measured in accordance with JIS K7121 (the same applies hereinafter).

When a heat resistant resin is added to the polyethylene resin, the compression resistance and electrolyte absorbability when the polyethylene multilayer microporous film is used as a separator for a battery can be improved. In the second porous layer, the heat-resistant resin is dispersed in the polyethylene resin in the form of a ball or spheroidal fine particles, and the fibril composed of the polyethylene resin forms a split with each fine particle as a core. It is preferable that the pores (fine crack-like voids of fine particles composed of a heat resistant resin are held in the center portion). The particle diameter of the spherical fine particles and the long diameter of the spheroidal fine particles are preferably 0.1 to 15 μm, more preferably 0.5 to 10 μm. When the pores formed by the fine crack-like voids as described above are formed in the second porous layer, the compression resistance and the electrolyte absorbability can be further improved.

When a crystalline resin having a melting point of less than 150 ° C or an amorphous resin having a Tg of less than 150 ° C is used, the resins are highly dispersed in the polyethylene-based resin, and fine particles having an appropriate diameter cannot be formed. Therefore, the crack formed by the resin fine particles as a core becomes small, and the compression resistance and the electrolyte absorbability are insufficient. The melting point of the heat resistant resin or the upper limit of the Tg is not particularly limited, and is preferably 350 ° C from the viewpoint of the mixing property of the polyethylene resin. The melting point or Tg of the heat resistant resin is preferably 160 to 260 °C.

The Mw of the heat resistant resin varies depending on the type of the resin, and is usually 1 × 10 ³ or more and 1 × 10 ⁶ or less, and more preferably 1 × 10 ⁴ or more and 8 × 10 ⁵ or less. When a heat resistant resin having an Mw of less than 1 × 10 ³ is used, it is highly dispersed in the polyethylene resin, and fine particles having an appropriate diameter cannot be formed. On the other hand, when a heat resistant resin of more than 1 × 10 ⁶ is used, kneading with a polyethylene resin becomes difficult.

Specific examples of the heat resistant resin include polyester, polymethylpentene [PMP or TPX (TORANSPANRET POLYMER X)], polypropylene, fluororesin, polyamine (PA, melting point: 215 to 265 ° C), Polyarylene sulfide (PAS), polystyrene (PS, melting point: 230 ° C), polyvinyl alcohol (PVA, melting point: 220 ~ 240 ° C), polyimine (PI, Tg: 280 ° C or more), polyfluorene Amine imine (PAI, Tg: 280 ° C), polyether oxime (PES, Tg: 223 ° C), polyether ether ketone (PEEK, melting point: 334 ° C), polycarbonate (PC, melting point: 220 ~ 240 ° C ), cellulose acetate (melting point: 220 ° C), cellulose triacetate (melting point: 300 ° C), polyfluorene (melting point: 190 ° C), polyether sulfimine (216 ° C), and the like. Among them, polyester, polymethylpentene, polypropylene, fluororesin, polyamine and polyarylene sulfide are preferred, and polyester, polymethylpentene and polypropylene are more preferred. The heat resistant resin is not limited to a single resin, and may be composed of a plurality of resin components. Hereinafter, polyester, polymethylpentene, polypropylene, fluororesin, polyamine, and polyarylene sulfide will be described in detail.

(a) Polyester polyester may include polybutylene terephthalate (PBT, melting point: about 160 to 230 ° C), polyethylene terephthalate (PET, melting point: about 250 to 270 ° C), Polyethylene naphthalate (PEN, melting point: about 272 ° C), polybutylene naphthalate (PBN, melting point: about 245 ° C), etc., of which PBT is preferred.

PBT is basically a saturated polyester composed of 1,4-butanediol and p-citric acid. However, a diol component other than 1,4-butanediol or a carboxylic acid component other than citric acid may be contained as a copolymerization component in the range of physical properties such as heat resistance, compression resistance, and dimensional stability. . Examples of such a diol component include ethylene glycol, diethylene glycol, pentaerythritol, and 1,4-cyclohexanol. Further, examples of the dicarboxylic acid component include isophthalic acid, sebacic acid, adipic acid, sebacic acid, and succinic acid. Specific examples of the PBT resin constituting PBT include, for example, a homogenous PBT resin sold under the trade name "TORAYCON" by TORAY AG. However, PBT is not limited to being composed of a single composition, and may be composed of a plurality of PBT resin components. The Mw of PBT is particularly preferably 2 × 10 ⁴ or more and 3 × 10 ⁵ or less.

(b) Polymethylpentene PMP is basically selected from the group consisting of 4-methyl-1-pentene, 2-methyl-1-pentene, 2-methyl-2-pentene, 3-methyl- A polyolefin composed of at least one of the group consisting of 1-pentene and 3-methyl-2-pentene, wherein a single polymer of 4-methyl-1-pentene is preferred. However, a small amount of other α-olefin other than methylpentene may be used in a range that does not impair the physical properties such as heat resistance, compression resistance, and dimensional stability of PMP. Other α-olefins other than methylpentene are preferably ethylene, propylene, butene-1, hexene-1, pentene-1, octene-1, ethyl acetate, methyl methacrylate, styrene, etc. . The melting point of PMP is usually 230 to 245 °C. The Mw of the PMP is particularly preferably 3 × 10 ⁵ or more and 7 × 10 ⁵ or less.

(c) The polypropylene polypropylene is not limited to a single polymer, and may be a copolymer with another olefin or a diene insofar as it does not impair the physical properties such as heat resistance, compression resistance, and dimensional stability. Other olefins are preferably ethylene or an alpha olefin. The carbon number of the α-olefin is preferably 4-8. The α-olefin having 4 to 8 carbon atoms may, for example, be 1-butene, 1-hexene or 4-methyl-1-pentene. The diene has a carbon number of 4 to 14. Examples of the diene having 4 to 14 carbon atoms include butadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene and the like. The content of other olefins or diolefins is preferably less than 10 mol% relative to 100 mol% of the propylene copolymer. The polypropylene may be a single substance or a composition containing two or more kinds of PP.

The Mw of the polypropylene is particularly preferably 1 × 10 ⁵ or more and 8 × 10 ⁵ or less. The molecular weight distribution (Mw/Mn) of the polypropylene is preferably from 1.01 to 100, more preferably from 1.1 to 50. The melting point of polypropylene is preferably 155 to 175 °C. The polypropylene having the above Mw, molecular weight distribution, and melting point is dispersed in the polyethylene resin so as to have fine particles having the above shape and particle diameter. Therefore, the fibrils constituting the microporous membrane form pores composed of fine crack-like voids which are split by the polypropylene microparticles.

(d) The fluororesin fluororesin may be exemplified by polyvinylidene fluoride (PVDF, melting point: 171 ° C), polytetrafluoroethylene (PTFE, melting point: 327 ° C), and tetrafluoroethylene/perfluoroalkyl vinyl ether copolymerization. (PFA, melting point: 310 ° C), tetrafluoroethylene / hexafluoropropylene / perfluoro (propyl vinyl ether) copolymer (EPE, melting point: 295 ° C), tetrafluoroethylene / hexafluoropropylene copolymer (FEP, melting point : 275 ° C), ethylene/tetrafluoroethylene copolymer (ETFE, melting point: 270 ° C), and the like.

The fluororesin is preferably PVDF. PVDF can also be used as a copolymer with other polyolefins (vinylidene fluoride copolymer). The vinylidene fluoride copolymer has a vinylidene fluoride unit of preferably 75 mass% or more, more preferably 90 mass% or more. Examples of the monomer copolymerized with vinylidene fluoride are hexafluoropropylene, tetrafluoroethylene, trifluoropropene, ethylene, propylene, isobutylene, styrene, vinyl chloride, vinylidene chloride, difluorochloroethylene, ethyl formate, acetic acid. Ethyl ester, ethyl propionate, ethyl butyrate, acrylic acid and its salt, methyl methacrylate, propylene methacrylate, acrylonitrile, methacrylonitrile, N-butoxymethyl acrylamide, acetic acid Propylene ester, isopropenyl acetate, and the like. The vinylidene fluoride copolymer is preferably a poly(hexafluoropropylene-vinylidene fluoride) copolymer.

(e) Polyamido PA to be selected from the group consisting of polyamine 6 (6-Nylon), polyamide 66 (6,6-nylon), polyamido 12 (12-nylon), and amorphous polyfluorene At least one of the groups consisting of amines is preferred.

(f) Polyarylene sulfide PAS is preferably polyphenylene sulfide (PPS, melting point: 285 ° C). The PPS may be either linear or bifurcated.

(g) When the total content of the polyethylene resin and the heat resistant resin is 100% by mass, the content of the heat resistant resin is preferably 3 to 30% by mass, more preferably 5 to 25% by mass. When the content is less than 3% by mass, the compression resistance and the electrolyte absorbability are insufficient. On the other hand, when the content is more than 30% by mass, the puncturing strength and the deformability at the time of compression are lowered.

(3) Filler

The filler may be exemplified by an inorganic filler and an organic filler. The inorganic filler may be cerium oxide, aluminum oxide, cerium oxide-alumina, zeolite, mica, clay, kaolin, talc, calcium carbonate, calcium oxide, calcium sulfate, barium carbonate, barium sulfate, magnesium carbonate, Magnesium sulfate, magnesium oxide, diatomaceous earth, glass powder, aluminum hydroxide, titanium dioxide, zinc oxide, satin-white, oxidized white clay, and the like. The inorganic filler may be used in combination of plural kinds instead of only one. Among them, it is preferred to use cerium oxide and/or calcium carbonate. The organic filler is preferably one composed of the above heat resistant resin.

The particle shape of the filler is not particularly limited. For example, a filler such as a spherical shape or a crushed shape can be appropriately selected, and a glassy shape is preferred. The volume average particle diameter of the filler is preferably from 0.1 to 15 μm, more preferably from 0.5 to 10 μm. The volume average particle diameter is measured in accordance with JIS Z8825-1 using a laser scattering type particle size distribution measuring apparatus. Surface-treated materials can also be used as fillers. The surface treatment agent for the filler may, for example, be various decane coupling agents, fatty acids (for example, stearic acid, etc.) or derivatives thereof.

By containing a filler together with the heat resistant resin, the electrolyte absorbability can be further improved. When it is estimated that the filler contains a filler, the fibrils composed of the polyethylene resin are cracked around the filler particles to form fine crack-like voids (fine pores), and the volume of the voids (fine pores) is further increased.

The content of the filler is preferably 0.1 to 5% by mass, more preferably 0.5 to 3% by mass, based on 100% by mass of the total of the polyethylene resin and the heat resistant resin. When the content is less than 0.1% by mass, the electrolyte absorbability is insufficient. On the other hand, when the content is more than 5% by mass, the puncturing strength and the deformability at the time of compression are lowered, and the filler falling during slitting is increased. If the powder produced by the loss of the filler is large, the multilayer microporous film product may have defects such as pinholes or black spots.

(4) Number of layers

The second porous layer is usually one layer, and may be formed into a plurality of layers as necessary. For example, a plurality of second porous layers having different compositions may be provided.

(5) The role of the second porous layer

When at least one second porous layer is interposed between the two surface layers, the compression resistance and electrolyte absorbability of the polyethylene multilayer microporous film become good.

(C) layer structure example and ratio of the first and second porous layers

Although not limited, the polyethylene multilayer microporous membrane may generally be a three-layer structure of the first porous layer/second porous layer/first porous layer. The ratio of the first porous layer to the second porous layer is not particularly limited, and can be appropriately set in accordance with the use of the multilayer microporous membrane. Usually, the mass ratio shown by (the polyethylene resin of the first porous layer) / (the total of the polyethylene resin of the second porous layer, the heat resistant resin, and the filler) is 70/30 to 10/90. For better, 60/40~20/80 is better.

[2] Method for producing polyethylene multilayer microporous membrane (A) First manufacturing method

The first production method of the polyethylene multilayer microporous film can be carried out by the following process: (1) (a) The polyethylene resin and the solvent for film formation are melt-kneaded to prepare a first melt kneaded product (first polyethylene) (b) simultaneously (b) melt-kneading each of the above polyethylene-based resin, heat-resistant resin, filler, and film-forming solvent to prepare a second melt-kneaded product (second polyethylene solution); (2) The first and second polyethylene solutions are extruded using a multilayer die, and the obtained extruded body is cooled to form a gel-like multilayer sheet; (3) an extension process; and (4) a solvent process for removing the film formation (5) and drying process. Moreover, (6) heat treatment process may be performed after (1) to (5) processes as necessary; (7) re-extension process; (8) cross-linking process by ionizing radiation; and (9) hydrophilization Processing process, etc.

(1) Process for preparing polyethylene solution (a) preparing a first polyethylene solution

The first polyethylene solution is prepared by melt-kneading a suitable solvent for film formation in a polyethylene resin. Various additives such as an antioxidant, an ultraviolet absorber, an anti-adhesive agent, a pigment, a dye, and an inorganic filler may be added to the first polyethylene solution as necessary within a range not impairing the effects of the present invention. For example, micronized citric acid is added as a pore former.

As the solvent for film formation, any of a liquid solvent and a solid solvent can be used. The liquid solvent may, for example, be an aliphatic or cyclic hydrocarbon such as decane, decane, decahydronaphthalene, p-xylene, undecane, dodecane or mobile paraffin, or a mineral oil fraction having a boiling point corresponding thereto. In order to obtain a gel-like sheet having a stable solvent content, it is preferred to use a non-volatile liquid solvent such as flowing paraffin. The solid solvent is preferably a melting point of 80 ° C or less, and examples of such a solid solvent include paraffin, hexadecanol, stearyl ester, dicyclohexyl phthalate and the like. A liquid solvent and a solid solvent may also be used in combination.

The liquid solvent viscosity is preferably in the range of 30 to 500 cSt at a temperature of 25 ° C, and more preferably in the range of 30 to 200 cSt. When the viscosity is less than 30 cSt, the discharge from the die of the resin solution is uneven and it is difficult to knead. On the other hand, when it is more than 500 cSt, it is difficult to remove the liquid solvent.

The homogeneously melt-kneading the first polyethylene solution is not particularly limited, and it is preferably carried out using a twin-screw extruder. Melt-kneading in a twin-screw extruder is suitable for preparing a high concentration polyethylene solution. The melt kneading temperature is preferably set to a melting point of the polyethylene composition contained in the polyethylene resin + 10 ° C to a melting point + 120 ° C. Specifically, the melt kneading temperature is preferably 140 to 250 ° C, and more preferably 170 to 240 ° C. The solvent for film formation may be added before the start of kneading, or may be added from the middle of the twin-screw extruder during kneading, and the latter is preferable. In order to prevent oxidation of the polyethylene resin during melt kneading, it is preferred to add an antioxidant.

The ratio (L/D) of the spiral length (L) to the diameter (D) of the twin-screw extruder is preferably in the range of 20 to 100, and more preferably in the range of 35 to 70. When L/D is less than 20, melt kneading may become insufficient. When L/D is more than 100, the residence time of melt kneading is excessively increased. The shape of the spiral is not particularly limited and may be a well-known shape. The inner diameter of the cylinder of the twin-screw extruder is preferably 40 to 100 mm.

When the total ratio of the polyethylene resin and the solvent for film formation in the first polyethylene solution is 100% by mass, the polyethylene resin is preferably 10 to 50% by mass, preferably 20 to 45% by mass. When the ratio of the polyethylene-based resin is less than 10% by mass, the expansion and shrinkage of the polyethylene solution are increased, and the formability and self-supportability of the gel-like molded body are lowered. On the other hand, when the ratio of the polyethylene resin is more than 50% by mass, the formability of the gel-like molded body is lowered.

(b) preparing a second polyethylene solution

The second polyethylene solution is prepared by adding the solvent for film formation to a polyethylene resin, a heat resistant resin, and a filler, followed by melt kneading. In the method of preparing the second polyethylene solution, the melt kneading temperature is preferably a melting point of the crystalline heat resistant resin or a Tg or more of the amorphous heat resistant resin in accordance with the type of the heat resistant resin, and is preferably in a polyethylene solution. The content of the solid component (polyethylene resin + heat resistant resin + filler) is preferably from 1 to 50% by mass, and is the same as the preparation method of the first polyethylene solution.

The melt-kneading temperature is equal to or higher than the melting point of the crystalline heat-resistant resin or the Tg of the amorphous heat-resistant resin in accordance with the type of the heat-resistant resin, and the heat-resistant resin can be dispersed in the form of fine particles in the polyethylene resin. The melt-kneading temperature is preferably a melting point of the crystalline heat-resistant resin or a Tg of the amorphous heat-resistant resin to a melting point of the polyethylene-based resin + 120 ° C or less. For example, when PBT (melting point: about 160 to 230 ° C) or polypropylene (melting point: 155 to 175 ° C) is used as the heat resistant resin, the melt kneading temperature is preferably 160 to 260 ° C, more preferably 180 to 250 ° C. When PMP (melting point: 230 to 245 ° C) is contained as the heat resistant resin, the melt kneading temperature is preferably 230 to 260 ° C. The solid content of the second polyethylene solution is preferably from 10 to 40% by weight.

(2) Process for forming a gel-like multilayer sheet

The melt-kneaded first and second polyethylene solutions are granulated from each extruder directly or via separate extruders, or temporarily cooled, and then passed through a plurality of extruders from the die. Simultaneous extrusion. The simultaneous extrusion method comprises the steps of combining the first and second polyethylene solutions in a layered manner in a die, extruding in a sheet form (adhesive method in the die), and from the respective molds. The method of extruding the first and second polyethylene solutions in a sheet form and sticking them outside the die (external die bonding method), because the productivity and the adhesion of the first and second polyethylene solutions are good, The former is better.

Simultaneous extrusion can also be carried out using either the flat die method or the blow molding method. When any method is applied to the inside of the die, a method of supplying the respective solutions to the respective manifolds of the multilayer die and laminating them at the inlet of the die lip (multi-tube method) or using each solution may be used. Any one of the methods (block method) of supplying the die to the die in a layered manner in advance. Since the manifold method and the block method itself are well known, detailed descriptions thereof are omitted herein. For example, a flat die for a multilayer and a blow molding die can use well-known ones. The gap of the multi-layer flat spray head is usually in the range of 0.1 to 5 mm. When the outside of the die is adhered by the flat die method, after the sheets extruded from the respective dies are adhered, it is also possible to perform pressure bonding by passing between a pair of rolls as necessary. In either of the above methods, the die is pressurized to a temperature of from 140 to 250 ° C during extrusion. The extrusion rate of the heated solution is preferably in the range of 0.2 to 15 meters per minute.

In this manner, the gel-like molded body was extruded from the lip by cooling to form a gel-like multilayer sheet. Cooling is preferably carried out at least at a rate of 50 ° C / min to a gelation temperature or lower. By such cooling, the resin phase (the polyethylene resin phase and the heat resistant resin phase) can be fixed by a structure in which the microphase is separated by a solvent for film formation. Cooling is preferably carried out below 25 °C. When the cooling rate is usually slowed down, the simulated cell unit becomes large, and the resulting high-order substructure of the gel-like multilayer sheet becomes thick, but the accelerated cooling rate becomes a dense cell unit. When the cooling rate is less than 50 ° C /min, the degree of crystallization increases, and it is not easy to become a gel-like multilayer sheet suitable for stretching. The cooling method may be a direct contact method in which the cooling solvent is directly contacted, a method in which it is brought into contact with the cooled roller, or the like, and a method of contacting the cooling roller is preferred.

(3) Extension process

The obtained gel-like multilayer sheet was stretched in at least a uniaxial direction. Since the gel-like multilayer sheet contains a solvent for film formation, it can be uniformly extended. The gel-like multilayer sheet may be extended at a prescribed magnification by a tenter method, a roll method, a blow molding method, a calendering method, or a combination thereof after heating. The extension can be uniaxially extended or biaxially extended. The biaxial extension may be any one of synchronous biaxial extension, sequential extension or multi-segment extension (for example, a combination of synchronous biaxial extension and sequential extension), wherein synchronous biaxial extension is particularly preferred.

The stretching ratio is preferably 2 times or more for uniaxial stretching, and preferably 3 times to 30 times. In the case of biaxial stretching, it is preferable to extend at least three times in any direction, the area ratio is 9 times or more, and the area magnification is preferably 25 times or more. When the area magnification is less than 9 times, a multilayer microporous film having high elasticity and high strength cannot be obtained due to insufficient elongation. On the other hand, when the area magnification is 400 times or more, there are restrictions on the stretching device, the stretching operation, and the like.

The stretching temperature is preferably set to a melting point of the polyethylene composition contained in the polyethylene resin + 10 ° C or less, and more preferably in the range of the crystal dispersion temperature to the lower melting point. When the stretching temperature is higher than the melting point of the polyethylene composition + 10 ° C, the resin is melted, and the molecular chain cannot be aligned by stretching. On the other hand, when it is lower than the crystal dispersion temperature, the softening of the resin is insufficient, and it is easy to break due to stretching, and it is not possible to extend at a high magnification. Here, the crystal dispersion temperature means a value obtained by measuring the temperature characteristics of dynamic viscoelasticity in accordance with ASTM D 4065. The ultrahigh molecular weight polyethylene and other polyethylenes have a crystal dispersion temperature of about 90 to 100 ° C and have a melting point of about 130 to 140 ° C. Therefore, the extension temperature is usually set in the range of 90 to 140 ° C, preferably in the range of 100 to 130 ° C.

It is also possible to provide a temperature distribution on the film thickness side in accordance with the desired physical properties, whereby a multilayer microporous film having more excellent mechanical strength can be obtained. This method is specifically described in Japanese Patent No. 3,347,854.

By the above extension, the polyethylene crystal plate layer is cleaved, and the polyethylene resin phase is refined to form a large number of fibrils. The obtained fibrils form a tertiary network structure (a structure in which three-dimensionally irregularly joined). In addition, the layer containing the heat-resistant resin has micropores composed of a heat-resistant resin as a core, and the fibrils are cleaved to form fine crack-like pores in which fine particles are held in the center portion.

(4) Removing the solvent process for film formation

Removal of the liquid solvent (washing) uses a washing solvent. Since the resin phase (polyethylene composition phase and heat resistant resin phase) is separated from the solvent for film formation, a porous multilayer film can be obtained when the film formation is dissolved. A well-known washing solvent can be used to remove the liquid solvent (washing). The washing solvent may, for example, be a saturated hydrocarbon such as benzene, hexane or heptane, a chlorinated hydrocarbon such as dichloromethane or carbon tetrachloride, or a diethyl ether or a second. Ethers such as ethers, ketones such as methyl ethyl ketone, chain fluorocarbons such as trifluoroethane, C ₆ F _{1 4} and C ₇ F _{1 6} , cyclic hydrofluorocarbons such as C ₅ H ₃ F ₇ , and C _a volatile solvent such as perfluoroether such as hydrofluoride ether such as ₄ F ₉ OCH ₃ or C ₄ F ₉ OC ₂ H ₅ or C ₄ F ₉ OCF ₃ or C ₄ F ₉ OC ₂ F ₅ . These washing solvents have a low surface tension (for example, 24 mN/m or less at 25 ° C). By using a washing solvent having a low surface tension, it is possible to suppress the shrinkage caused by the surface tension of the gas-liquid surface during the drying of the microporous network structure after the washing, and therefore, it is possible to have a high porosity and penetration. Multilayer microporous membrane.

The washing of the stretched gel-like multilayer sheet can be carried out by a method of immersing in a washing solvent, a method of washing a solvent, or a combination thereof. The washing solvent is preferably used in an amount of 300 to 30,000 parts by mass based on 100 parts by mass of the multilayer film after stretching. It is preferred to wash the strip with a washing solvent to a residual amount of the liquid solvent of less than 1% by mass of the initial amount added.

(5) Drying process of multilayer film

The polyethylene multilayer microporous film obtained by removing the solvent for stretching and film formation is dried by a heat drying method or an air drying method. The drying temperature is preferably not less than the crystal dispersion temperature of the polyethylene composition contained in the polyethylene resin of the first porous layer, and more preferably 5 ° C or more lower than the crystal dispersion temperature. When the dry multi-layer microporous film is 100% by mass (dry weight), the residual washing solvent is preferably 5% by mass or less, and more preferably 3% by mass or less. When the drying is insufficient, the porosity of the multilayer microporous film is lowered and the penetrability is deteriorated at the time of post-treatment, which is not preferable.

(6) Heat treatment process

It is preferred to heat-treat the dried multilayer film. By heat treatment, the crystal can be stabilized and the layer can be made uniform. The heat treatment method may use a heat setting treatment and/or a heat relaxation treatment. The heat setting treatment is performed in a temperature range from the crystal dispersion temperature to the melting point of the polyethylene composition contained in the polyethylene resin of the first porous layer. The heat setting process is carried out by a tenter method, a roller method or a calendering method.

The heat relaxation treatment can be carried out using a conveyor belt or a blown type heating furnace in addition to the above. The heat relaxation treatment is carried out at a temperature lower than the melting point of the polyethylene composition contained in the polyethylene resin of the first porous layer, preferably in a temperature range of from 60 ° C to -10 ° C. By such a heat relaxation treatment, a high-strength multilayer microporous film having good penetration can be obtained. Further, it is also possible to carry out a combination of a plurality of heat setting treatments and heat relaxation treatments.

(7) Re-extension process

It is preferable to further extend the multilayer film after drying in at least the uniaxial direction. The extension can be carried out while heating the film by the same tenter method as described above. The extension can be either uniaxial or biaxial. The biaxial extension may be either synchronous biaxial extension or sequential extension, wherein synchronous biaxial extension is preferred.

The temperature to be further extended is preferably set to be equal to or lower than the melting point of the polyethylene composition contained in the polyethylene resin of the first porous layer, and more preferably in the range from the crystal dispersion temperature to the melting point or lower. When the re-extension temperature is higher than the melting point, the compression resistance is lowered, and when the width direction (TD) is extended, the variation in physical properties (especially, air permeability) in the sheet width direction is increased. On the other hand, when the re-expansion temperature is lower than the crystal dispersion temperature, the softening of the resin is insufficient, and the film is easily broken during stretching and cannot be uniformly extended. Specifically, the extension temperature is usually set in the range of 90 to 135 ° C, preferably in the range of 95 to 130 ° C.

The magnification in the uniaxial direction of the re-expansion is preferably 1.1 to 2.5 times, whereby the compression resistance of the multilayer microporous film can be further improved. For example, when uniaxially extending, the length direction (MD) or the width direction (TD) is 1.1 to 2.5 times. In the case of biaxial stretching, the MD direction and the TD direction are each 1.1 to 2.5 times, and the MD direction and the TD direction may be different from each other, but the same is preferable. When the magnification is less than 1.1 times, the compression resistance cannot be sufficiently improved. On the other hand, when the magnification is more than 2.5 times, it is not preferable because the possibility of film rupture is increased and the dimensional stability is lowered. The re-expansion ratio is preferably 1.1 to 2.0 times.

(8) Membrane crosslinking process

The multilayer microporous membrane after drying can be subjected to a crosslinking treatment by irradiating ionizing radiation such as α rays, β rays, γ rays, and electron beams. When irradiating an electron beam, it is preferable to use an electron beam amount of 0.1 to 100 Mrad, and an acceleration voltage of 100 to 300 kV is preferable. The melting temperature of the polyethylene multilayer microporous membrane treated by crosslinking is increased.

(9) Hydrophilization treatment process

The dried multilayer microporous membrane can be subjected to a hydrophilization treatment. The hydrophilization treatment can be carried out by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer is grafted to be preferred after the crosslinking treatment.

As the surfactant treatment, any of a nonionic surfactant, a cationic surfactant, an anionic surfactant, or a two-ionic surfactant may be used, and among them, a nonionic surfactant is preferred. The multilayer microporous membrane is immersed in a solution obtained by dissolving a surfactant in water or a low carbon number alcohol such as methanol, ethanol or isopropyl alcohol, or coating the solution on a multilayer microporous membrane by a doctor blade method.

(B) Second manufacturing method

The second manufacturing method is the same as the first manufacturing method except that after the heat-fixing treatment of the extended gel multilayer sheet is carried out, the solvent for film formation is removed, and the other processes are the same. The heat setting treatment can be the same as described above.

(C) Third manufacturing method

The third manufacturing method is the same as the first manufacturing method except that the extended multilayer film is brought into contact with the hot solvent before and/or after the solvent for film formation is removed, and the other processes are the same. Therefore, only the hot solvent treatment process will be described.

The hot solvent treatment is preferably carried out before the solvent for film formation is removed. The solvent which can be used for the heat treatment is preferably the solvent for liquid film formation described above, and more preferably liquid paraffin. However, the solvent for heat treatment may be the same as or different from those used in the production of the polyethylene solution.

The method of treating the hot solvent is not particularly limited as long as it can be brought into contact with the hot solvent, and there is, for example, a method in which the stretched multilayer film is directly contacted with the hot solvent (hereinafter, unless otherwise specified in advance, it is abbreviated as " The direct method") is a method in which the extended multilayer film is directly contacted with a cold solvent and heated (hereinafter, unless otherwise specified in advance, referred to as "indirect method"). The direct method includes a method of immersing a stretched multilayer film in a hot solvent, a method of spraying a hot solvent on a stretched multilayer film, a method of coating a hot solvent on a stretched multilayer film, etc., wherein a dipping method is preferred, Perform uniform processing. The indirect method may be carried out by immersing the stretched multilayer film in a cold solvent, spraying a cold solvent on the stretched multilayer film, applying a cold solvent on the stretched multilayer film, bringing it into contact with a heat roll, heating in an oven, and dipping. Medium method in hot solvents.

The pore size and porosity of the multilayer film can be varied by varying the temperature and processing time of the hot solvent treatment process. The temperature of the hot solvent is preferably a temperature in the range of not less than the crystal dispersion temperature of the polyethylene composition contained in the polyethylene resin of the first porous layer and not more than +10 ° C. Generally, the temperature of the hot solvent is preferably 110 to 140 ° C, more preferably 115 to 135 ° C. The contact time is preferably between 0.1 second and 10 minutes, preferably from 1 second to 1 minute. When the temperature of the hot solvent is less than the temperature of the crystal dispersion and the contact time is less than 0.1 second, the hot solvent treatment has almost no effect, and the penetrability cannot be improved. On the other hand, when the temperature of the hot solvent is higher than the melting point + 10 ° C and the contact time is more than 10 minutes, there will be It is not preferable that the strength of the film is lowered and the film is broken.

After the multilayer film is subjected to a hot solvent treatment, the residual heat treatment solvent is washed and removed. Since the washing method can be the same as the method of removing the solvent for film formation described above, the description is omitted. Of course, when the hot solvent treatment is performed before removing the solvent for film formation, the heat treatment solvent can be removed even when the solvent for film formation is removed.

By the above thermal solvent treatment, the fibrils formed by the extension are deformed into veins, and the dried fibers become relatively coarse. Therefore, the pore diameter is relatively large, and a multilayer microporous membrane excellent in strength and penetration can be obtained. Here, "corn vein fibrils" means that the fibrils are composed of coarse dry fibers and fine fibers attached to the outer side, and the fine fibers form a complex network structure. Further, the hot solvent treatment process is not limited to the third manufacturing method, and may be applied to the second manufacturing method. That is, the thermally stretched extended multilayer film may be contacted with the hot solvent before/or after the film formation is removed by the second manufacturing method.

(D) Fourth manufacturing method

The fourth manufacturing method has (i) a process of preparing the first and second polyethylene solutions in the same manner as described above; (ii) extruding the first and second polyethylene solutions by a die, and cooling the obtained a process for extruding a molded body to form a gel-like sheet; (iii) a process for extending each of the obtained gel-like sheets; and (iv) a process for removing a solvent for film formation from each of the extended gel-like sheets; v) a drying process; and (vi) a process of alternately joining the resulting first and second polyethylene microporous membranes. Moreover, after (i) to (vi), the (vii) re-extension process, (viii) heat treatment process, (ix) cross-linking process by ionizing radiation, (x) may be performed as necessary. Hydrophilization treatment process, etc. Further, heat setting treatment may be performed after the process (iv). The above thermal solvent treatment process may also be carried out before and/or after the process (iv).

The process (vi) of alternately joining the obtained first and second polyethylene microporous films will be described below. The joining method is not particularly limited, and a thermal bonding method is preferred. The thermal bonding may be exemplified by thermal law, pulse thermal law, ultrasonic bonding method, etc., of which thermal law is preferred, and a hot roll method is also preferred. However, it is not limited to the hot roll method. The heat roll method is performed by laminating the first and second polyethylene microporous membranes between a pair of heat rolls or between a heat roll and a receiving stage to perform heat sealing. The temperature and pressure at the time of heat sealing are not particularly limited as long as the first and second polyethylene microporous membranes are sufficiently adhered and the properties of the obtained multilayer microporous membrane are not lowered, and can be appropriately set. The heat sealing temperature is usually 90 to 135 ° C, preferably 90 to 115 ° C. The ratio of the first and second porous layers can be adjusted by adjusting the respective thicknesses of the first and second polyethylene microporous membranes.

[3] Characteristics of polyethylene multilayer microporous membrane

The polyethylene multilayer microporous membrane obtained by the above method has the following physical properties.

(1) 20 to 400 seconds / 100 cubic centimeters of air permeability (converted film thickness 20 micrometers) air permeability of 20 to 400 seconds / 100 cubic centimeters, when using polyethylene multilayer microporous membrane as a battery separator, the battery The capacity is increased and the cycle characteristics of the battery are also improved. When the air permeability is less than 20 sec / 100 m ^ 3 , the temperature inside the battery rises and the shutdown can be sufficiently performed.

(2) When the porosity of 25 to 80 seconds is less than 25%, the polyethylene multilayer microporous membrane does not have good gas permeability. On the other hand, when it is more than 80%, when a polyethylene multilayer microporous film is used as a separator for batteries, the strength is insufficient and the risk of short-circuiting of the electrode increases.

(3) When the puncturing strength of 3,000 mN/20 μm or more is less than 3,000 mN/20 μm, when the microporous film is used as a battery separator, the electrode may be short-circuited. The plucking strength is preferably greater than 3,500 mN / 20 microns.

(4) Tensile breaking strength at 80,000 kPa or more Tensile breaking strength When the longitudinal direction (MD) and the width direction (TD) are both 80,000 kPa or more, there is no fear of film breakage. The tensile breaking strength is preferably 100,000 kPa or more in both the longitudinal direction (MD) and the width direction (TD).

(5) Tensile elongation at break of 100% or more Tensile elongation at break When the longitudinal direction (MD) and the width direction (TD) are both 100% or more, there is no fear of film breakage.

(6) When the heat shrinkage ratio of 10% or less is 10 minutes after exposure at 105 ° C, and the longitudinal direction (MD) and the width direction (TD) are both greater than 10%, a polyethylene multilayer microporous film is used as a battery. In the case of the separator, the separator may shrink due to the heat generated by the battery, and the end portion may be short-lived. The heat shrinkage ratio is preferably 8% or less in both the longitudinal direction (MD) and the width direction (TD).

(7) When the film thickness change rate after heat compression is 30% or more at a pressure of 2.2 MPa (22 kgf/cm ² ), when the film thickness change rate after heating and compression at 90 ° C for 5 minutes is 30% or more, polyethylene multilayer is used. When the microporous membrane is used as a separator for a battery, absorption of electrode expansion is good. The film thickness change rate is preferably 40% or more.

(8) 700 seconds / 100 cubic centimeters of heat-compressed air permeability (converted film thickness of 20 microns) under the pressure of 2.2MPa (22kgf / cm ² ), the air permeability after heating at 90 ° C for 5 minutes (to reach the breathable When the polyethylene multilayer microporous film is used as a separator for a battery, the battery capacity is large and the cycleability of the battery is also good when the temperature is 700 seconds/100 m ^ 3 / 20 μm or less. The air permeability is preferably 600 seconds / 100 cubic meters / 20 microns or less.

(9) The absorption amount of the electrolyte of 0.3 g/g or more is immersed in the electrolyte at room temperature, and the average membrane mass of the absorbed electrolyte is 0.3 g/g [the amount of the electrolyte (g) / the membrane before absorption) Quality (g) above. The amount of the electrolytic solution is preferably 0.4 g/g or more.

[4] Battery separator

The separator for a battery comprising the polyethylene multilayer microporous film can be appropriately selected depending on the type of the battery, and the film thickness is preferably 5 to 50 μm, more preferably 10 to 35.

[5]Battery

The battery using the separator composed of the polyethylene multilayer microporous film of the present invention is not particularly limited, and is particularly preferably used for a lithium secondary battery. As the secondary lithium battery using the separator composed of the polyethylene multilayer microporous film of the present invention, a well-known electrode and an electrolytic solution can be used. Further, the structure of the secondary lithium battery using the separator composed of the polyethylene multilayer microporous film of the present invention may be a well-known structure.

[Examples]

The invention is explained in more detail below, but the invention is not limited to these examples.

Example 1

(Preparation of the first polyethylene solution) by mass average molecular weight (Mw) of 2.0 × 10 ⁶ ultrahigh molecular weight polyethylene (UHMWPE) 20% by mass, and Mw of 3.5 × 10 ⁵ high density polyethylene (HDPE) 80% by mass 0.2 parts by mass of hydrazine [methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate] methane as an oxidizing agent in 100 parts by mass of the polyethylene composition To prepare the mixture. Among them, the melting point of 135 ° C and the crystal dispersion temperature of the polyethylene (PE) composition composed of UHMWPE and HDPE were 100 °C.

The Mw of UHMWPE and HDPE was measured by gel permeation chromatography (GPC) using the following conditions.

. Measuring device: GPC-150C manufactured by Waters Corporation. Column: Shodex UT806M manufactured by Showa Denko Co., Ltd. Column temperature: 135 ° C. Solvent (mobile phase): o-dichlorobenzene. Solvent flow rate: 1.0 ml / min. Sample concentration: 0.1 wt% (dissolution conditions: 135 ° C / hour). Injection volume: 500μl. Detector: differential graftmeter (RI detector) manufactured by Waters Corporation. Calibration curve: A calibration curve obtained by using a monodisperse polyethylene standard sample, which was prepared using a constant from a constant.

30 parts by mass of the obtained mixture was placed in a twin-screw extruder (inner diameter: 58 mm, L/D = 42, strong kneading type), and 70 parts by mass of mobile paraffin was supplied from the side feeder of the twin-screw extruder [35cst (40 ° C)], melt-kneaded at 230 ° C and 250 rpm to prepare a first PE solution (PE solution for surface layer).

(Preparation of the second polyethylene solution) 0.2 parts by mass of the above oxidizing agent and 2.0 in 100 parts by mass of the resin composed of 15% by mass of UHMWPE, 655% by mass of HDPE, and 20% by mass of PBT (Mw: 3.5 × 10 ⁴ ) A mass fraction of cerium oxide powder (volume average particle diameter: 1 μm) was used to prepare a resin composition. 25 parts by mass of the obtained composition was placed in another twin-screw extruder of the same type as described above, and 75 parts by mass of mobile paraffin [35 cst (40 ° C)] was supplied from the side feeder of the twin-screw extruder under the same conditions. The second PE solution (PE solution for the inner layer) was prepared by melt-kneading.

(film formation) The obtained surface layer and inner layer PE solution were supplied from each twin-screw extruder to a three-layer T die, and the surface layer PE solution/inner layer PE solution/surface layer was laminated in this order. The PE solution was extruded as a molded body (mass ratio: PE solution for the surface layer / PE solution for the inner layer / PE solution for the surface layer = 27.25 / 45.5 / 27.25). The extruded molded body was pulled while being cooled by a cooling roll adjusted to 0 ° C to form a gel-like three-layer sheet. The biaxially stretched gel-like three-layer sheet was synchronized by a tenter stretching machine at a rate of 5 times in the longitudinal direction (MD) and the width direction (TD) at 115 °C. The obtained stretched film was fixed on an aluminum frame of 20 cm × 20 cm. The mixture was immersed in dichloromethane which had been tempered to 25 ° C, and washed while shaking at 100 rpm for 3 minutes. The obtained film was air-dried at room temperature, and subjected to a heat relaxation treatment at 125 ° C for 10 minutes using a tenter stretching machine to prepare a polyethylene multilayer microporous film.

Example 2

A polyethylene multilayer microporous film was produced in the same manner as in Example 1 except that the heat-treated film was re-extruded at a temperature of 125 ° C in a width direction (TD) of 1.2.

Example 3

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the film which was biaxially stretched was subjected to heat setting treatment at 123 ° C for 10 minutes.

Example 4

In addition to the film that has been biaxially stretched, it is fixed to a frame plate [size: 20 cm × 20 cm, made of aluminum], and after immersing in a flowing paraffin bath for 3 seconds, it is washed after being adjusted to 130 ° C, and the embodiment. 2 was carried out in the same manner to obtain a polyethylene multilayer microporous film.

Example 5

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that calcium carbonate was used as the filler.

Example 6

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that polymethylpentene (TPX, Mw: 5.2 × 10 ⁵ ) was used as the heat resistant resin.

Example 7

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that polypropylene (PP, Mw: 5.3 × 10 ⁵ ) was used as the heat resistant resin.

Example 8

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the longitudinal direction (MD) was the direction of re-expansion.

Example 9

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the resin composition of the inner layer polyethylene solution was 15% by mass of UHMWPE, 75% by mass of HDPE, and 10% by mass of PBT.

Example 10

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the resin composition of the polyethylene resin solution of the inner layer was 15% by mass of UHMWPE, 55% by mass of HDPE, and 30% by mass of PBT.

Comparative example 1

The same composition and the same concentration of PE solution as in the first PE solution of Example 1 were prepared. A single-layer polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the obtained PE was extruded from the T die.

Comparative example 2

A PE solution for a surface layer was prepared in the same manner as in Example 1 except that 2.0 parts by mass of a cerium oxide powder (volume average particle diameter: 1 μm) was added per 100 parts by mass of the polyethylene composition. The PE solution for the inner layer was prepared in the same manner as in Example 1 except that the cerium oxide powder was not added. A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the obtained PE layer solution for the surface layer and the inner layer was used.

Comparative example 3

The PE solution having the same composition and the same concentration as the second PE solution of Example 1 was prepared. A single-layer polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the obtained PE was extruded from the T die.

Comparative example 4

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the resin composition of the inner layer polyethylene solution was 15% by mass of UHMWPE, 50% by mass of HDPE, and 35% by mass of PBT.

Comparative Example 5

A polyethylene multilayer microporous film was produced in the same manner as in Example 2 except that the content of cerium oxide in the polyethylene solution for the inner layer was 7 parts by mass based on 100 parts by mass of the total of the PE composition and the PBT.

The physical properties of each of the microporous membranes obtained in Examples 1 to 10 and Comparative Examples 1 to 5 were measured by the following methods. The results are shown in Tables 1 and 2.

(1) Mean film thickness (micrometer) at any position in the longitudinal direction of the multilayer microporous film, the film thickness in the width direction (TD) was measured in the width direction (TD) by a contact thickness meter at intervals of 5 mm, and the average film thickness was measured value.

(2) Air permeability (sec / 100 cc / 20 [mu] m) multilayer microporous film to a film thickness T _1, ie, the air permeability measured by JIS P8117 according to P _1, according to the _{formula: P 2 = (P 1 ×} 20 ) / T ₁ , converted to a gas permeability P ₂ at a film thickness of 20 μm.

(3) The porosity (%) is determined by the mass method.

(4) Puncture strength (mN/20 μm) was measured using a diameter of a spherical surface (curvature radius R: 0.5 mm) at the front end, and a multilayer microporous film having a film thickness T ₁ at a speed of 2 mm/sec. load. The maximum load measured value L _{1 is} converted into a maximum load L ₂ at a film thickness of 20 μm by the formula: L ₂ = (L ₁ × 20) / T ₁ as the puncturing strength.

(5) Tensile breaking strength and tensile elongation at break A thin rectangular test piece having a width of 10 mm was used in accordance with ASTM D 882.

(6) Thermal Shrinkage Ratio (%) The shrinkage ratio in the longitudinal direction (MD) and the width direction (TD) after exposure for 8 hours at 105 ° C was measured three times to obtain an average value.

(7) Closing temperature Using a heat/stress/deformation measuring device (manufactured by SEIKO Electronics Co., Ltd., TMA/SS6000), a microporous film sample of 10 mm (TD) × 3 mm (MD) was placed at a load of 2 g. The sample was stretched in the longitudinal direction, and the temperature was raised from room temperature at a rate of 5 ° C / minute, and the temperature at the inflection point near the melting point was observed.

(8) Melting temperature (° C.) Using a heat/stress/deformation measuring apparatus described above, a microporous film sample of 10 mm (TD) × 3 mm (MD) was stretched by a load of 2 g toward the length of the sample. The temperature was raised from room temperature at a rate of 5 ° C/min, and the temperature at which the film was broken by melting was measured.

(10) Film-forming property A multilayer microporous film (length 500 m) wound up on a reel was attached to a slitter, and was unwound at a speed of 50 m/min, and cut into a forward direction. In the two halves, the obtained 500-meter longitudinal sections were slid into contact with the fixing rods and then taken up on a reel. The powder attached to the fixed rod was recovered and its mass was measured.

(11) Air permeability and film thickness change rate after heating and compression The microporous film sample was sandwiched between one of the high smooth surfaces and the pressure plate, and was 2.2 MPa (22 kgf/cm ² by a press machine). The pressure was heated and compressed at 90 ° C for 5 minutes, and the average film thickness and air permeability (reaching the gas permeability value) were measured by the above method. The film thickness change rate was calculated by taking the average film thickness before heat compression to 100%.

(12) Electrolyte absorption rate Using a dynamic surface tension measuring device (DCAT21 manufactured by Yinghong Seiki Co., Ltd., with a precision electronic balance), the microporous membrane was immersed in an electrolyte kept at 18 ° C (electrolyte: LiPF6, electrolyte concentration: 1 Mo Ear/L, solvent: ethylene carbonate + dimethyl carbonate), the quality of the investigation is increased, and the absorption amount of the average sample mass [the increase in the mass of the film (g) / the film mass before the absorption (g)] is taken as the absorption. Speed indicator. The absorption rate [absorption amount (g/g)] of the film of Comparative Example 1 is represented by a relative ratio of 1.

(13) Absorbance of electrolyte solution A 50-volume microporous membrane sample (width: 60 mm, length: 2 m) and a roll-like laminate (small gel-like roll) without an electrode were placed in a glass. In the test tube (diameter: 18 mm, height: 65 mm), the electrolyte solution was injected into the electrolyte solution using a liquid injection device (VD102i, manufactured by FINE FLOW Co., Ltd.), and the sample was immersed at room temperature for 1 minute, and then taken out, and the quality of the investigation was increased. The amount of absorption of the average sample mass [the amount of increase in film mass (g) / the film mass before absorption (g)] was calculated.

Note: (1) Mw represents the mass average molecular weight. (2) The amount of addition of UHMWPE+HDPE+ heat resistant resin at 100% by mass. (3) Mass ratio of PE solution (PE solution for surface layer/PE solution for inner layer/PE solution for surface layer). (4) The MD system indicates the longitudinal direction, and the TD system indicates the width direction.

Note: (1) Same as Table 1. (2) Same as Table 1. (3) The amount of addition of UHMWPE+HDPE+ heat resistant resin at 100% by mass. (4) Mass ratio of PE solution (PE solution for surface layer/PE solution for inner layer/PE solution for surface layer). (5) The MD system indicates the longitudinal direction, and the TD system indicates the width direction.

From Table 1, the inner layers of Examples 1 to 10 are composed of a second porous layer composed of a polyethylene composition, a heat resistant resin, and an inorganic filler, since polyethylene is composed on both sides of the inner layer. The first porous layer is not only excellent in balance of penetrability, mechanical properties, dimensional stability, shutdown characteristics, and melting, but also has compression resistance (deformability at the time of compression and penetration after compression). ) and electrolyte absorption (absorption rate and absorption) are also excellent, and the amount of filler peeling during slitting is very small.

On the other hand, the film of Comparative Example 1 was only a single layer film composed of a PE composition, and when compared with Examples 1 to 10, the air permeability (air permeability) after heating and compression and the electrolyte absorbability were inferior. Therefore, it is expected that the battery capacity or cycle characteristics of the film of Comparative 1 as a separator for a battery may be insufficient, for example, the capacity may be lowered prematurely during repeated charge and discharge. In the film of Comparative Example 2, since the inorganic filler was contained in the surface layer instead of the inner layer, when compared with Examples 1 to 10, the electrolyte solution was inferior in absorbability, and many powders were generated due to the fall of the inorganic filler. The film of Comparative Example 3 contains a single layer film of a PE composition, a heat resistant resin, and a filler. When compared with Examples 1 to 10, the film thickness change of heat compression is small, the deformability is poor, and the inorganic filler is peeled off. Many powders. The film of the comparative example 4 has a PBT content of more than 30% by mass based on 100% by mass of the total of the PE composition and the heat resistant resin in the polyethylene solution for internal use, and the puncture strength and compression when compared with Examples 1 to 10. The deformation is poor. In the film of the comparative example 5, the content of cerium oxide is more than 5 parts by mass based on 100% by mass of the total of the PE composition and the heat resistant resin in the polyethylene solution for internal use, and the puncturing is compared with Examples 1 to 10. The strength and the deformability at the time of compression are poor, and many powders are produced due to the fall of the inorganic filler.

Claims (4)

A polyethylene multilayer microporous membrane comprising at least three layers of a polyethylene multilayer microporous membrane characterized by having: (a) a first porous layer composed of a polyethylene resin and formed a surface layer of at least two sides; and (b) a second porous layer containing a polyethylene resin, a heat-resistant resin having a melting point or a glass transition temperature of 150 ° C or higher, and an inorganic filler, at least one layer sandwiched between the two skin layers The polyethylene-based resin of the first and second porous layers is made of ultrahigh molecular weight polyethylene having a mass average molecular weight of 5 × 10 ⁵ or more, and Mw of 1 × 10 ⁴ or more to less than 5 × 10 ⁵ The composition of the high-density polyethylene is at least one selected from the group consisting of polyester, polymethylpentene, and polypropylene. The polyethylene multilayer microporous film of claim 1, wherein the polyester is composed of polybutylene terephthalate. A separator for a battery, which is characterized by comprising a polyethylene multilayer microporous membrane according to claim 1 or 2. A battery characterized by using a polyethylene multilayer microporous film according to claim 1 or 2 as a battery separator.